This invention relates to the construction of a magnet assembly having a central bore suitable for reception of a patient, as in a magnetic resonance imaging (MRI) system, wherein the assembly comprises an array of coaxial segmented ellipses of which each segment is fabricated of a mosaic of bricks of magnetic material. The magnetization vector orientation in the ellipses is determined in a unique fashion to minimize external field and provide homogeneous center field. To reduce overall weight of magnetic material, each section has an elliptical shape, and full uncut bricks are employed throughout each segment with the exception of the segment surface facing the bore due to a truncation of one or more of the bricks to obtain a desired homogeneity of the magnetic field in the bore.
MRI systems are widely used today. A characteristic feature in the construction of such a system is the use of a large permanent magnet assembly which encloses a central cylindrical bore. Typically, the bore has a circular cross section of sufficient diameter to receive a patient, and the magnet assembly provides a homogeneous field within the bore as is required for the production of magnetic resonance images. An example of such construction of the magnet assembly is taught in Gluckstern et al, U.S. Pat. No 4,580,098, wherein the assembly is composed of a set of circular ring dipole sections which, in turn, are subdivided into segments composed of bricks of magnetic material. In order to provide the desired configuration to each segment, and for providing the desired strength and distribution of magnetic field, the bricks in peripheral regions of a segment are cut, as by a taper, to provide a smooth edge surface. In addition, each segment is totally filled with the bricks.
Such a configuration of magnet assembly is relatively large and heavy as compared to other types of imaging instruments. The large mass of magnetic material is disadvantageous, not only from a point of view of transportation and assembly, but also from a point of view of cost. In addition, the tapering of the bricks is disadvantageous in the sense that the tapering entails additional manufacturing procedures and, hence cost.